1. Field of the Invention
The present invention is directed to a telecommunications satellite system on quasi-stationary orbits comprising a plurality of earth satellites having an elliptical orbit and which appear seen from an earth station at nearly the same point in space in cyclical sequence and which relieve one another with respect to radio range.
2. Description of the Prior Art
In telecommunications satellites on quasi-stationary orbits whose ground paths are theoretically congruent and comprise one or more closed loops, a system of satellites is involved in which the chronological graduation and a satellite switching strategy provide that geostationary, position loops which are uninterruptedly traversed by satellites of changing identity, arise.
Telecommunications satellite systems comprising geostationary position loops in which the two orbital arms of the loops meet and satellite collisions are avoided by the inaccuracies in orbit (probability of about 10.sup.-9 per encounter) are disclosed in the German OS 31 45 207 and in a publication "Loopus erschliesst dem Satellitenfunk eine neue Dimension", NTZ Archive No. 12/1983.
Given application of a digital transmission method via the satellite broadcast link, a few bits in the transmitted radio signal can be lost at the time of satellite switching when this is not prevented by suitable measures. One possibility is to assure that the overall radio link is somewhat longer after the switching than before switching. As a result thereof, the last signal tail of the transmission via the satellite departing the loop position is repeated at the head of the signal of the transmission via the satellite entering into the loop. This bit sequence doubly received in the receiving ground station can be employed for synchronization and a seamless joining of the transmitted information in order to guarantee interruption-free signal flow via the interface to the terestrial telecommunications network.
Digital transmission methods which operate on the principle set forth above are disclosed in the German OS 34 37 317.
Precise studies have shown, however, that given the known orbital geometry and constellation, this digital transmission:
can either not be continually carried out given all switching instances; or the transmission capacity cannot be optimally exploited.
If one wishes to reliably operate the described transmission method via geostationary position loops with optimum transmission capacity, the geometry of the radio links must be taken into consideration. The participating, follow-up ground stations track the active satellites. The sectional volume of all antenna main lobes of these ground stations is composed of a double cone around the active satellites whose one tip is directed in the direction of the earth and whose other tip is directed in the opposite direction, as shall be set forth below with reference to FIG. 2. In order to work free of interruption in terms of radio engineering, it is necessary that the successor or following satellite assume the telecommunications function within the double cone.
Given the known orbit constellation in which the two satellites theoretically meet, the two requirements of "switching in the double cone" and "successor satellite at a greater distance than the active satellite" are not possible in all switchovers and given an optimally-designed radio link. It is not inconsiderable in a portion of the instances, either the successor satellite will have to be switched outside of the double cone (gain/fade) or the signal repetition due to lengthening of the radio link will not occur (disturbance of the freedom from interruption).